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Negative pressure reading

Posted: Thu Feb 16, 2017 4:21 am
by Sarvesh Sadana
While testing out a new vacuum gauge (my old one broke), I got a result of -2.0 mtorr on the controller.
The convectron gauge I purchased is a Granville Phillips 275071 convectron gauge tube with a minimum pressure reading of 10e-4 torr. The resistance values of the pins check out, as does the pressure reading at atmosphere. The controller is a terrenova 906A controller from Duniway. None of the manuals say anything regarding negative values.

How should I interpret this value?

Re: Negative pressure reading

Posted: Thu Feb 16, 2017 5:14 am
by John Futter
See the two trim pots on the front panel
These Vac and atmosphere need to be adjusted with each new gauge head
Normally you adjust the atmoshere first to read rioght at atmosphere then with a known vacuum better than the gauge can handle you adjuct the vac control to zero. It would pay to do a couple of iterations to get it exactly right
hope this helps

Re: Negative pressure reading

Posted: Thu Feb 16, 2017 5:57 am
by Sarvesh Sadana
Yes, I know. I calibrated it at atmosphere, but the actual pressure inside the chamber is unknown to me, and I have no other gauges that I can use to calibrate it.

The last pressure reading I took with a working gauge was .7 mtorr, but I have made a few modifications to the chamber since then, and the gauge I used wasn't calibrated as well.

Re: Negative pressure reading

Posted: Thu Feb 16, 2017 11:54 am
by Dennis P Brown
A simple and useful way to calibrate is to get another gauge that is known to be accurate (not often an option since one needs access to such a gauge.) Another method, and one that depends on you having a high vac system with no leaks (out gassing is not important here) is to run your high vac (should easily reach the mid/low 10^-5 torr or better) and use that as a zero for your gauge (I've done both methods. I generally now just use the later since it is easy.)